The neutron star ( NS ) merger GW170817 was followed over several days by optical-wavelength ( “ blue ” ) kilonova ( KN ) emission likely powered by the radioactive decay of light r -process nuclei synthesized by ejecta with a low neutron abundance ( electron fraction Y _ { e } \approx 0.25 - 0.35 ) .
While the composition and high velocities of the blue KN ejecta are consistent with shock-heated dynamical material , the large quantity is in tension with the results of numerical simulations .
We propose an alternative ejecta source : the neutrino-heated , magnetically-accelerated wind from the strongly-magnetized hypermassive NS ( HMNS ) remnant .
A rapidly-spinning HMNS with an ordered surface magnetic field of strength B \approx 1 - 3 \times 10 ^ { 14 } G and lifetime t _ { rem } \sim 0.1 - 1 s can simultaneously explain the velocity , total mass , and electron fraction of the blue KN ejecta .
The inferred HMNS lifetime is close to its Alfvén crossing time , suggesting global magnetic torques could be responsible for bringing the HMNS into solid body rotation and instigating its gravitational collapse .
Different origins for the KN ejecta may be distinguished by their predictions for the emission in the first hours after the merger , when the luminosity is enhanced by heating from internal shocks ; the latter are likely generic to any temporally-extended ejecta source ( e.g .
magnetar or accretion disk wind ) and are not unique to the emergence of a relativistic jet .
The same shocks could mix and homogenizes the composition to a low but non-zero lanthanide mass fraction , X _ { La } \approx 10 ^ { -3 } , as advocated by some authors , but only if the mixing occurs after neutrons are consumed in the r -process on a timescale \gtrsim 1 s .